1. Field of the Invention
The invention relates to hybrid electronic circuitry by which is meant electronic circuitry partitioned into plural integrated circuits formed on separate chips and requiring further chip-to-chip interconnections for completion of the circuitry. The invention has application to the case where large numbers of chip-to-chip interconnections are to be made with a fine pitch and with delicacy to avoid injury to chips constituted of fragile semiconductor materials. The invention has special application to the focal planes of solid state cameras in which interconnections are provided between a delicate Mercury Cadmium Telluride (MCT) chip bearing an array of photosensors and more rugged silicon integrated circuits providing readout.
2. Prior Art
Wire bonds are the conventional method of interconnecting integrated circuits. In MCT chips bearing an array of photosensors, sensitive to infrared, they suffer from several limitations. The first limitation is a bonding pitch limitation of 2.5 mils (currently) in single level bonding. This limitation restricts wire bonding to hybridization of square arrays, line arrays and small depth time delay integration (TDI) arrays. Wire bonds cannot achieve the fine pitches required in TDI arrays 6 to 8 pixels deep, for which there is substantial demand. This is true even in arrays using large pixels, i.e., 3 mils.times.5 mils. Assuming that half the runs, of a 6 deep TDI array, are brought out on each of two sides of the array, the bonding pitch interval is 5 mils/3-1.67 mils. This application thus requires a more complex alternative two level bonding technique. The pitch requirements of an 8 deep TDI array, which are also of practical interest, are well beyond the capabilities of current wire bonding.
A second limitation with wire bonding is the damaging stresses exerted on delicate chips. The damage produced on relatively delicate MCT IR sensing chips by wire bonding ranges from catastrophic mechanical damage to minor reductions in detector performance. The bonding may produce physical damage to the bond pads and metallization runs on the MCT chip surface. Cracking and chipping of the underlying MCT chip may also occur. Additionally, even when there is little or no physical damage apparent on the surface of the chip, the bonding forces may produce internal stresses and damage to the detector crystal that results in degraded infrared sensing performance. To avoid this damage the bond pads are usually placed as far from the active photosensors as practical.
The more rugged silicon material is not equally subject to this kind of damage, and wire bonding as practiced with chips of silicon material is accordingly not well suited to bonding the more delicate semiconductor materials such as MCT, GaAs, InSb, etc.
A known interconnection technique for a delicate material which avoids wire bonding is the beam lead technique. It provides both freedom from bonding damage and a finer interconnection pitch interval than wire bonds can provide.
In this known interconnection technique, the metal runs from each pixel on the top surface of an MCT chip are extended and increased in thickness to form metal beams. Next the MCT chip is thinned from the back side and the MCT material directly under the metal beams is removed to form cantilevered beam leads. The under surface of the MCT chip is then bonded to the silicon readout chip.
The thinning of the MCT chip in this known technique permits the fine beam leads to traverse the substantial distance from the upper surface of the MCT chip to the bonding pads on the silicon readout chip coplanar with the lower surface of the MCT chip. The thinning of the MCT chip reduces the total length required of the beam lead, and reduces the amount of deformation required of the beam lead to effect the bond at the lower plane, thereby increasing the reliability of the interconnection.
This known technique avoids damaging forces on the MCT chip, but requires thinning the MCT chip to a few mils and requires subsequent handling of a very fragile chip with a consequent reduction in yield. The present approach seeks to improve over this known beam lead approach.